Ohmic processing of Arenga pinnata sap: determining the interaction effects of storage time, voltage gradient, and natural preservatives

Arenga pinnata sap is prone to quality deterioration during ambient holding, which may affect thermal processing performance. This study examined how sap without preservative and with natural preservatives (Curcuma longa rhizome and Pterospermum sp. root), pre-heating storage time (0, 4, and 8 h), and voltage gradient (10–20 V cm⁻¹) jointly influence the physicochemical stability and ohmic-heating behaviour of Arenga pinnata sap. A multifactorial design was used to evaluate electrical conductivity, heating rate, pH, total soluble solids (TSS), turbidity, and invert sugar. Storage time emerged as the dominant factor affecting both quality and processing behaviour: extended holding promoted early deterioration, reflected by a decline in pH and an increase in turbidity, and this coincided with higher electrical conductivity and faster heating. The temperature–time and conductivity–temperature relationships were well described by regression models (R² > 0.99). In contrast, preservative type and voltage gradient more strongly influenced sugar-related responses (TSS and invert sugar) and contributed to turbidity differences across operating conditions. Both preservatives helped maintain TSS and invert sugar, but C. longa better preserved sap clarity, whereas Pterospermum sp. tended to increase turbidity while providing better pH retention. These findings highlight that minimising pre-heating storage and selecting voltage gradients aligned with the preservative strategy are critical to maintain sap stability while enabling efficient ohmic processing.